This invention relates to a process for separating or recovering polyvinyl alcohol (hereinbelow it is referred to as PVA) from its solution.
PVA is a water-soluble polymer having various desirable characteristics and has been used in various fields in recent years. Thus, a considerable amount of PVA has been discharged from factories in the form of waste water. PVA has a small Biochemical Oxygen Demand (BOD) and rarely appears to cause problems in waste water treatment; however, it has a large Chemical Oxygen Demand (COD) and is liable to cause a public nuisance to the human health. Therefore, it is desirable to remove PVA from the waste water as completely as possible. Thus, it became necessary to separate and remove PVA even from the waste water containing a small amount of PVA, for example, 0.003 to 3 percent by weight of PVA.
In general, in order to treat the waste water which is liable to cause the public nuisance on account of organic compounds contained therein, various processes have been used such as an activated sludge process, a contact process, or an oxidizing pool process. These processes are based on the principle that the organic compounds in the waste water can be decomposed through the action of aerobic organisms. However, PVA can not be decomposed through the action of common aerobic organisms. Therefore, PVA can not be removed by these processes. An attempt was made to discover a specific microorganism which has a capacity of decomposing PVA and to remove PVA through the action of said microorganism. Another attempt was made to separate PVA from aqueous medium by dissolving it into a liquid hydrocarbon medium. However, these attempts could not be of practical use as each was found to be expensive and uneconomical.
On the other hand, it was contemplated that PVA might be separated by salting out a PVA solution. However, a large amount of salt is required in order to precipitate PVA by salting out from a dilute PVA solution such as from the waste water. Use of such a large amount of salt makes it difficult to carry out the salting process on an industrial scale. Thus, it became necessary to find a process in which PVA can be effectively precipitated by use of less amount of salt.
The reason why a large amount of salt is required in the conventional process is explained in more detail as follows. Anhydrous sodium sulfate is taken as an example of salt in the following explanation, because it is generally used as a salt in the conventional salting process. About 100 milliliters of 30 percent aqueous sodium sulfate solution is needed for separating PVA from 100 milliliters of a 2 percent aqueous PVA solution. Thus, 15 parts of anhydrous sodium sulfate (Na.sub.2 SO.sub.4) is needed for 1 part of PVA. On the other hand, it is said that the higher a valency of an ion, the bigger a coagulating power of the ion. However, when borax is added to an aqueous PVA solution, PVA is not precipitated, but is only gelled. In other words, when borax alone is added to an aqueous PVA solution, PVA only absorbs water and remains in the solution in a jelly form. Therefore, borax is called as a gelling agent for PVA and is understood as being different from the salting agent. Thus, when inorganic salts are added to an aqueous PVA solution in order to coagulate PVA in the solution, a large amount of the salts is sometimes needed, or the salts do not cause any precipitation, depending on the kinds of the salts. Because of these facts the salting process by which PVA is precipitated did not lead to be carried out in an industrial scale.
The inventor have now discovered that a mixture of borax and sodium sulfate has an excellent property of precipitating PVA, even when the mixture of the borax and sodium sulfate are added to a PVA solution in a relatively small amount in total. The inventors have also discovered, on one hand, that salts of boric acid, not only borax, have generally similar effects on precipitating PVA, and on the other hand, that the other component to be mixed with the salts of boric acid is not restricted to the sodium sulfate, but may be an inorganic acid salt of metal which belongs to the first group or the second group in the periodic table, an inorganic ammonium salt or an inorganic aluminum salt. The inventors have further discovered that water insoluble salts may be used for the inorganic acid salt when alkali such as sodium hydroxide etc. is added. This invention has been completed on the basis of above findings.
This invention relates to a process for separating PVA from its solution which comprises adding (1) a salt of boric acid and (2) an inorganic acid salt of a metal selected from the class consisting of the first group and the second group in the periodic table, an inorganic ammonium salt, or an aluminum salt to an aqueous PVA solution.
In the invention, two kinds of inorganic salts as defined in above (1) and (2) are used, in which the salt of boric acid defined in above (1) is hereinbelow referred to as a gelling agent, and the inorganic acid salt as defined in above (2) is hereinbelow referred to as a salting agent, and a mixture of both gelling agent and salting agent is hereinbelow referred to as a coagulating agent.
It is desirable in carrying out the invention that a great number of minute foams are present in the precipitates of PVA formed from an aqueous PVA solution by adding the coagulating agent. The foams, or finely divided gas bubbles suspended in a liquid, may be introduced into the aqueous solution either by bubbling air into the solution or by agitating the solution vigorously.
Though PVA is satisfactorily separated in a batch system using the coagulating agent according to the invention, a further contrivance is needed for separating PVA in a continuous system. The reason is that, although PVA can be precipitated by adding the coagulating agent when a vessel provided with the conventional stirrer is used, formed precipitates often grow to bulky masses and the masses adhere to walls of the vessel, so that it becomes difficult or impossible to carry out continuously such a process. The same difficulty occurs when efforts are made to crush the masses in vessels having conventional blades. Thus, PVA could not be continuously separated in a vessel provided with the conventional stirrer.